Tag Archives: freezable

The DataContext property in WPF is extremely handy, because it is automatically inherited by all children of the element where you assign it; therefore you don’t need to set it again on each element you want to bind. However, in some cases the DataContext is not accessible: it happens for elements that are not part of the visual or logical tree. It can be very difficult then to bind a property on those elements…

Let’s illustrate with a simple example: we want to display a list of products in a DataGrid. In the grid, we want to be able to show or hide the Price column, based on the value of a ShowPrice property exposed by the ViewModel. The obvious approach is to bind the Visibility of the column to the ShowPrice property:

The message is rather cryptic, but the meaning is actually quite simple: WPF doesn’t know which FrameworkElement to use to get the DataContext, because the column doesn’t belong to the visual or logical tree of the DataGrid.

We can try to tweak the binding to get the desired result, for instance by setting the RelativeSource to the DataGrid itself:

But none of these workarounds seems to work, we always get the same result…

At this point, it seems that the only viable approach would be to change the column visibility in code-behind, which we usually prefer to avoid when using the MVVM pattern… But I’m not going to give up so soon, at least not while there are other options to consider 😉

The solution to our problem is actually quite simple, and takes advantage of the Freezable class. The primary purpose of this class is to define objects that have a modifiable and a read-only state, but the interesting feature in our case is that Freezable objects can inherit the DataContext even when they’re not in the visual or logical tree. I don’t know the exact mechanism that enables this behavior, but we’re going to take advantage of it to make our binding work…

The idea is to create a class (I called it BindingProxy for reasons that should become obvious very soon) that inherits Freezable and declares a Data dependency property: